1. Field of the Invention
The present invention generally relates to a technical field of digital radio communications, and especially relates to a quadrature modulation system, and a transmitter equipped with the quadrature modulation system.
2. Description of the Related Art
Radio signals are transmitted using a wide band in a radio communications system like IMT 2000, and the bandwidth will be made even greater in the next-generation mobile communications systems. In order to transmit such signals, it is commonly practiced that a compound baseband signal is converted into a signal of an intermediate-frequency (IF) band, and then converted into a signal of a radio frequency (RF) band. However, in order to sufficiently reduce unwanted RF components originated by the frequency conversion stages, parts such as a band pass filter are required to provide steep and flat properties such that only a desired signal is allowed to pass. As the band is further widened in the future, high-performance parts are required, which tends to increase equipment size and product cost. For this reason, a direct RF modulation technique for directly converting a baseband signal into an RF band attracts attention. Conventional modulation techniques as described above are disclosed by Patent References 1 through 10, for example.
[Patent reference 1] JPA 6-350658
[Patent reference 2] JPA 7-123123
[Patent reference 3] JPA 7-177188
[Patent reference 4] JPA 8-116343
[Patent reference 5] JPA 2000-270037
[Patent reference 6] JPA 2001-339452
[Patent reference 7] JPA 2002-27007
[Patent reference 8] JPA 5-207080
[Patent reference 9] JPA 2002-77285
[Patent reference 10] JPA 8-97873
[Problem(s) to be solved by the Invention]
However, when the direct RF modulation technique is adopted, characteristic differences from part to part, secular change, and the like of analog parts tend to cause errors to arise between an inphase signal component and its orthogonal signal component. Accordingly, when the signal components are input to a quadrature modulator, there is a possibility that an image signal (unnecessary signal) will be generated and output in addition to a desired signal (refer to FIG. 1), causing degradation of the quality of the signal.